1. Field of the Invention
This invention relates to the art of substrate micro-fabrication and, more particularly, to processed substrate discharge and transfer from an electrostatic chuck.
2. Related Arts
Micro-fabrication of substrates is a well know art employed in, for example, fabrication of semiconductors, flat panel displays, light emitting diodes (LED's), solar cells, etc. Various steps in micro-fabrication may include plasma-assisted processes, (such as, plasma enhanced chemical vapor deposition, reactive ion etching, etc.) performed inside a reaction chamber, into which a process gas is introduced. A radio frequency (RF) source is inductively and/or capacitively coupled to the reaction chamber to energize the process gas to form and maintain a plasma. Inside the reaction chamber, an exposed substrate is supported by a chuck, and held at a fixed position on the chuck by some kind of clamping force.
One conventional type of chuck is an electrostatic chuck (ESC), which uses electrostatic forces to hold or clamp a substrate during processing. A DC electrode, coupled to a high-voltage DC source induces opposite polarity electrostatic charges between the ESC and the wafer to generate the electrostatic clamping force.
After processing is done, the substrate needs to be removed or “de-chucked” from the chuck. To de-chuck, the high-voltage DC source is turned off. However, as residual charges tend to remain on the entire backside of the substrate or at portions of the backside of the substrate, often the substrate fails to separate from the wafer, and/or breaks into pieces or is otherwise damaged during lifting.
In prior arts, various techniques have been attempted to facilitate de-chucking by supplying a reverse polarity discharging voltage to the DC electrode, and/or providing an outlet for the residual charge. In one of the existing methods, a peak current flowing through the chuck during lifting the substrate at the end of a plasma process is measured to control the amplitude and/or duration of the reverse polarity discharging voltage in the next run. In another existing methods, a capacitance detector is used for measuring the capacitance between the ESC electrode and the substrate, and calculates a reverse polarity discharging voltage based on the capacitance measured by the capacitance detector. Thus, magnitude of the reverse polarity discharging voltage fluctuates with various factors, such as RF power applied during wafer processing, surface status of the ESC, etc.
Lift pins are used in some conventional ESC chucks to raise the substrate from the surface of the ESC chuck, so that a robotic arm can reach beneath the substrate, and transport the processed substrate away. In one existing method, a lift pin is made conductive and is grounded to provide a discharging path to the residual charge. The lift pin may incorporates a RF filter and/or a resistor along the discharging path. In another existing method, a lift pin is grounded via a switch and a variable resistor. For reference, readers are directed to U.S. Pat. No. 6,790,375 to Howald et al., US publication 2008/0218931 to Hsu et al., U.S. Pat. No. 5,900,062 to Loewenhardt et al., and U.S. Pat. No. 6,646,857 to Anderson et al.
What is needed is de-chucking systems and methods of de-chucking that reduce de-chucking failure by discharging residual charges from a processed substrate, and optimize the subsequent de-chucking process by using gathered data from a prior de-chucking run.